The oil and gas industry typically conducts comprehensive evaluation of underground hydrocarbon reservoirs prior to their development. Formation evaluation procedures generally involve collection of formation fluid samples for analysis of their hydrocarbon content, estimation of the formation permeability and directional uniformity, determination of the formation fluid pressure, and many other parameters. Measurements of such parameters of the geological formation are typically performed using many devices including downhole formation testing tools.
Recent formation testing tools generally comprise an elongated tubular body divided into several modules serving predetermined functions. A typical tool may have a hydraulic power module that converts electrical into hydraulic power; a telemetry module that provides electrical and data communication between the modules and an uphole control unit; one or more probe modules collecting samples of the formation fluids; a flow control module regulating the flow of formation and other fluids in and out of the tool; and a sample collection module that may contain various size chambers for storage of the collected fluid samples. The various modules of such a tool can be arranged differently depending on the specific testing application, and may further include special testing modules, such as NMR measurement equipment. In certain applications the tool may be attached to a drill bit for logging-while-drilling (LWD) or measurement-while drilling (MWD) purposes.
Among the various techniques for performing formation evaluation (i.e., interrogating and analyzing the surrounding formation regions for the presence of oil and gas) in open, uncased boreholes have been described, for example, in U.S. Pat. Nos. 4,860,581 and 4,936,139, assigned to the assignee of the present invention. An example of this class of tools is Schlumberger's MDT™, a modular dynamic fluid testing tool, which further includes modules capable of analyzing the sampled fluids. In a variant of the method the sampler is located between a pair of straddle packers to isolate a section of a well which can then be fractured and sampled.
To enable the same sampling in cased boreholes, which are lined with a steel tube, sampling tools have been combined with perforating tools. Such cased hole formation sampling tools are described, for example, in the U.S. Pat. No. 7,380,599 to T. Fields et al. and further citing the U.S. Pat. Nos. 5,195,588; 5,692,565; 5,746,279; 5,779,085; 5,687,806; and 6,119,782, all of which are assigned to the assignee of the present invention. The '588 patent by Dave describes a downhole formation testing tool which can reseal a hole or perforation in a cased borehole wall. The '565 patent by MacDougall et al. describes a downhole tool with a single bit on a flexible shaft for drilling, sampling through, and subsequently sealing multiple holes of a cased borehole. The '279 patent by Havlinek et al. describes an apparatus and method for overcoming bit-life limitations by carrying multiple bits, each of which are employed to drill only one hole. The '806 patent by Salwasser et al. describes a technique for increasing the weight-on-bit delivered by the bit on the flexible shaft by using a hydraulic piston.
Another perforating technique is described in U.S. Pat. No. 6,167,968 assigned to Penetrators Canada. The '968 patent discloses a rather complex perforating system involving the use of a milling bit for drilling steel casing and a rock bit on a flexible shaft for drilling formation and cement.
U.S. Pat. No. 4,339,948 to Hallmark discloses an apparatus and methods for testing, then treating, then testing the same sealed off region of earth formation within a well bore. It employs a sealing pad arrangement carried by the well tool to seal the test region to permit flow of formation fluid from the region. A fluid sample taking arrangement in the tool is adapted to receive a fluid sample through the sealing pad from the test region and a pressure detector is connected to sense and indicate the build up of pressure from the fluid sample. A treating mechanism in the tool injects a treating fluid such as a mud-cleaning acid into said sealed test region of earth formation. A second fluid sample is taken through the sealing pad while the buildup of pressure from the second fluid sample is indicated.
Methods and tools for performing downhole fluid compatibility tests include obtaining an downhole fluid sample, mixing it with a test fluid, and detecting a reaction between the fluids are described in the co-owned U.S. Pat. No. 7,614,294 to P. Hegeman et al. The tools include a plurality of fluid chambers, a reversible pump and one or more sensors capable of detecting a reaction between the fluids. The patent refers to a downhole drilling tool for cased hole applications.
In the light of above known art it is seen as an object of the present invention to improve and extend methods of sampling downhole formations, particular “tight” formations of low permeability. Prominent examples of such tight formations are shale gas formations.
The sampling of tight shale gas formation, which can be very thick, poses a problem to existing sampling tools and methods as the reservoir fluids are not easily extracted from the formation. Hence it is not easy to determine whether a newly drilled section of tight formation is potentially productive or not, even though important technical and economic decisions depend on correct answers to this question.
Among the methods used are formation sampling with a straddle packer configuration, underbalanced drilling, which allows for influx from the reservoir into the drilled well, and exploration fracturing. The latter is an extensive fracturing process on par in cost and complexity with normal fracturing operations.
However none of the known methods are entirely satisfactory as formations can be too tight for the typical one square meter of wellbore wall between the pair of packers to produce a significant sample. Underbalanced drilling on the other hand is typically vastly more expansive and dangerous compared to conventional drilling and the reservoir depth of any gas influx is difficult to determine with the necessary precision. There is further the suspicion that tight formations may not release trapped gas until fractured.
Therefore it is seen as the only reliable method to fully fracture the formation for a comprehensive test. However fracturing thick formations along their entire length becomes a very expansive operation as shale gas formation may stretch for more than 1000 m and considering that exploration fracturing may only cover 20 m to 50 m intervals at a time and at a cost of several million dollars per interval. The problem of deriving new and improved testing methods is therefore one of great importance for tight formations.